The present invention relates to an exhaust gas purification system and an exhaust gas purification method which make it possible to prevent an unburned hydrocarbon from being adsorbed into an exhaust gas purification unit, such as a diesel particulate filter (“DPF”) or a catalyst, for purifying exhaust gas from an internal combustion engine and maintain the temperature of the exhaust gas purification unit always in a controllable state.
An exhaust gas purification system for an internal combustion engine, such as a diesel engine, mounted on a vehicle is provided with a device using a DPF unit for removing particulate matter (PM) in exhaust gas, or an exhaust gas purification device including a lean NOx trap catalyst (LNT catalyst) unit in which a NOx storage reduction catalyst, a selective reduction catalyst (SCR), or the like is supported for removing nitrogen oxides (NOx) by reduction in a lean atmosphere.
For the DPF unit, the following method for recovering a PM collection ability is employed. Specifically, when a PM collection limit is approached, forced regeneration is performed in which the temperature of the exhaust gas is raised to combust and remove PM collected in the DPF unit. Thus, the PM deposited in the DPF unit is combusted and removed. For raising the temperature of the exhaust gas in the forced regeneration, post injection is performed by in-cylinder fuel injection after a piston passes a top dead center, and unburned fuel injected by the post injection is fed into an exhaust passage, or hydrocarbons (HC), which are unburned fuel, are fed into an exhaust passage by injecting fuel directly into the exhaust path from a fuel injection device provided in the exhaust passage. The hydrocarbons are oxidized in an oxidation catalyst unit of the exhaust gas purification device, and the exhaust gas whose temperature is raised by the heat of the oxidation is sent to the DPF unit. Thus, the temperatures of the DPF unit and the PM collected in the DPF unit are raised to be not lower than a temperature at which the PM can be combusted. In this manner, PM is combusted and removed.
Meanwhile, for the exhaust gas purification unit using a NOx storage reduction catalyst, the following method is employed. Specifically, in a rich control for recovering a NOx storage ability, post injection or direct fuel injection into an exhaust passage is employed, and hydrocarbons are combusted in an oxidation catalyst unit or the like to consume oxygen in the exhaust gas and raise the temperature of the exhaust gas. Then, the exhaust gas is fed to the NOx storage reduction catalyst to convert the surface state of the NOx storage reduction catalyst to a rich and high-temperature state. Thus, NOx occluded in the NOx storage reduction catalyst is released, and also the released NOx is reduced by a catalytic action.
In addition, for an exhaust gas purification unit using a hydrocarbon selective reduction catalyst (HC-SCR catalyst) for removing NOx, the following method is employed. Specifically, hydrocarbons such as fuel are injected to an upstream side of the hydrocarbon selective reduction catalyst by direct fuel injection into an exhaust passage, and the hydrocarbons are used as a reducing agent to reduce NOx in the exhaust gas by a selective reduction catalyst.
As described above, in some exhaust gas purification methods for exhaust gas purification systems provided with exhaust gas purification devices for purifying exhaust gas, hydrocarbons, which are fuel for internal combustion engines such as light oil, are fed to the upstream side of the exhaust gas purification devices, which are provided with an oxidation catalyst unit, a DPF unit, or a lean NOx catalyst unit in which a NOx storage reduction catalyst, a hydrocarbon selective reduction catalyst, or the like is supported, under predetermined hydrocarbon feed conditions preset respectively for the exhaust gas purification units at their respective timings and in their respective feed amounts.
However, there is a problem that when hydrocarbons are added in a case where the exhaust gas temperature is not higher than the activation start temperature (light-off temperature) of hydrocarbons in each of the exhaust gas purification units, the hydrocarbons are not combusted on the catalyst, but adsorbed into the catalyst.
Moreover, by way of many experiments, the present inventors have found that, even in a case where the exhaust gas temperature is near the activation start temperature and hydrocarbons are being combusted, if the flow rate of the exhaust gas increases because of acceleration of the vehicle on which an internal combustion engine is mounted or the like, the flame of the combustion is blown out and a misfiring phenomenon occurs in some cases.
It is also found that the HC-poisoned catalyst due to adsorption of the unburned hydrocarbons undergoes a phenomenon in which when a state where hydrocarbons are not combusted is shifted to a state where the exhaust gas temperature rises to a temperature not lower than the activation start temperature, the hydrocarbons are combusted at once and the state becomes uncontrollable. In this state, the temperature of the catalyst rises abnormally, causing degradation of the catalyst or dissolution loss of the DPF. In addition, this phenomenon may also occur in a desulfurization control of a lean NOx trap catalyst (LNT catalyst).
In this respect, as described in Japanese patent application Kokai publication No. 2011-153591 (Patent Document 1), an exhaust gas treatment method and device for an internal combustion engine are proposed in order to prevent abnormal combustion in a DPF filter occurring in an exhaust gas treatment device provided with a pre-stage oxidation catalyst and a DPF filter in a exhaust gas passage of an internal combustion engine when the state of the internal combustion engine changes from a high-load state to a low-load state. Specifically, in the method and device, when operation which may cause abnormal combustion in the DPF is detected, an intake air throttle valve is fully opened to increase the exhaust gas flow rate. Thus, the DPF filter device is cooled by taking heat away by sensible heat of the exhaust gas, and the oxygen concentration around the DPF filter is lowered by continuing late post injection, so that abnormal combustion of PM collected in the DPF filter is suppressed.
In this case, the abnormal combustion of PM collected by the DPF filter is suppressed, which is different from the above-described abnormal combustion of hydrocarbons. However, the above-described abnormal combustion of hydrocarbons is highly likely to cause abnormal combustion of PM collected in a DPF filter. This indicates the importance of suppression of the abnormal combustion in an exhaust gas purification unit.